This invention relates to X-ray tube protective circuits and more particularly to improved means for automatically preventing X-ray exposures under overload conditions.
Conventional high powered X-ray tubes normally include tungsten filament encased in a cathode cup which is mounted a short distance away from a rotating tungsten anode. The anode, in turn, is connected to a motor armature and bearing assembly with the entire structure mounted within a glass envelope of the X-ray tube. The tube is placed such that the motor armature and that portion of the glass envelope surrounding it are within the motor stator winding. When this stator winding is energized, the anode rotates so that during the X-ray exposure new areas of the anode are brought within the electron beam cross section. The thermal capacity, that is the maximum X-ray exposure is determined by the energy levels per exposure, which is a function of peak power expressed in terms of voltage (kV) .times. current (mA) and, the exposure time (seconds), the area on the anode subtended by the electron beam as well as the shape, and finally the speed of rotation of the anode.
In an effort to obtain the maximum output per exposure of the X-ray tubes utilized, each manufacturer attempts to rate its tube at the maximum possible value per exposure such that the anode is brought almost to the point of melting during each X-ray exposure. The X-ray tube manufacturers accordingly publish curves called "X-ray tube anode rating curves" which describe the maximum exposure for each X-ray tube under the various conditions of its operation.
Conventional protective circuits which have been used for many years in X-ray generators are based on generating each of the separate functions of maximum instantaneous power versus time by means of a constant voltage power supply source feeding voltage divider switch decks which are mounted on the same shaft of the X-ray exposure time switch. If the X-ray generator for example operates with two X-ray tubes and if each X-ray tube has two different focal spot sizes and the anodes are permitted to rotate at both standard speed (3000 rpm) or ultra speed (9000 rpm) eight separate switch decks would be required. If one were to replace one tube type with another tube type, it became necessary that appropriate switch decks had to be replaced in the field.
A more recent attempt to overcome the maintenance problem associated with the multiple switch decks is taught in U.S. Pat. No. 3,838,285, entitled "X-ray Tube Anode Protective Circuit", M.P. Siedband, et al. That invention eliminates the multiple switch deck requirement by making use of a single resistive voltage divider switch simulating an imperically derived generalized tube rating curve which is then modified, i.e. tilted and/or offset to fit the particular tube used.
Another means of providing a protective circuit is taught in U.S. Pat. No. 3,746,862, entitled "Protective Circuit For X-ray Tube and Method of Operation", D.F. Lombardo, et al. which discloses a signal generating circuit for developing a limit signal which varies in value with respect to time in accordance with a maximum tube rating signal together with a programming circuit for developing a program signal having a value representative of a preselected signal to be applied to the X-ray tube and a comparator circuit for developing an interrupt signal if the program signal exceeds the value of the limit signal.